Ultra Fire WF500, stock bulb/battery combo problems

I have a Ultra fire WL500 with stock xenon bulb, using two 18650 batteries. bought the light, batteries, charger all on ebay, all ultra fire brand stuff.

I keep burning out bulbs, my flashlight fanatic buddy tells me its too much voltage? Why does Ultrafire sell a non working combo? What can I do to remedy this? Different size batteries?

I also need to go battery shopping, I have one battery that comes off the charger and measures only 1.1 volts. It was my understanding these are protected cells, so I cannot discharge them too far? What else kills these batteries? Isn't the Ultrafire charger supposed to peak charge and shut off?

I am a newb to all of this, what I explained is about the extent of my knowledge.

Thanks.

Although i don't own a WF-500, I have used that bulb in a maglite mod and run it successfully at 9v. A pair of 18650s will run at 8.4v in series hot off the charger and, as you have correctly assumed, should not burn that bulb out. How fast is it burning through the bulb? I read somewhere that they don't have the longest lifespan, but if it's instaflashing or burning out in a few hours it's probably a quality control issue. You generally get what you pay for with these things, and although they are are quite bright they probably aren't built all that well. You might want to look into an LED module for it like this:

http://www.dealextreme.com/details.dx/sku.7039

It won't be as bright as the Xenon, since the stock bulb is pretty bright for it's price point, but the LED module will run longer and be more reliable.